SINGLE AND DUAL LABELING OF CYTOTOXIC TARGET-CELLS - COMPARISON OF 3 RADIOACTIVE-TRACERS, [S-35] METHIONINE, [SE-75] SELENOMETHIONINE, AND CHROMIUM-51

[Se-75]selenomethionine ((SeM))-Se-75 has been shown to provide several advantages over (Na2CrO4)-Cr-51 (Cr-51) labelling of metabolizing target cells: high labelling efficiency and low spontaneous release of (SeM)-Se-75-labelled target cells permit improved monitoring of cytotoxicity due to extended effector/target ratios in short- and long-term assays. Unfortunately, (SeM)-Se-75 will soon be difficult to obtain. Therefore we studied the suitability of [S-35]Methionine ((SM))-S-35 as a substitute for (SeM)-Se-75. Furthermore, we explored the potential of dual labelling of suspension target cells applying combinations of (SM)-S-35 and Cr-51 or (SeM)-Se-75 and Cr-51. (SM)-S-35 is a suitable substitute for (SeM)-Se-75 retaining most of the advantages of (SeM)-Se-75 labelling. Although considerably higher labelling of cells is possible we obtained the most efficient labelling with 100-400 kBq/ml of (SM)-S-35 or (SeM)-Se-75 resulting in a relatively high uptake (3-15 cpm/cell) and very low spontaneous release (1-2%/h) up to 24 h. This permits short- and long-term cytotoxic assays and the use of low numbers of target cells (1 x 10(3)) providing increased cytotoxic sensitivity with reduced amounts of effector cells. Suitable dual labelling of target cells with (SM)-S-35 plus Cr-51 or (SeM)-Se-75 plus Cr-51 documented convincingly identical release kinetics for (SM)-S-35 and (SeM)-Se-75 but partially discordant ones for Cr-51. Depending on the target cell used dual labelling permits discrimination and monitoring of different cytotoxic or release mechanisms in cellular cytotoxicity.